1. Field of the Invention
The present invention relates to a driving method suitable for a display which uses a display material such as a liquid crystal and in which display pixels are arranged in a matrix form, and particularly to an alternating current driving method of a liquid crystal panel.
2. Description of the Related Art
In recent years, a technique for manufacturing a semiconductor device in which a semiconductor thin film is formed on an insulating substrate, such as a thin film transistor (TFT), has been rapidly developed. The reason thereof resides in that the demand for a liquid crystal display device (typically, an active matrix type liquid crystal display device) has been increased.
The active matrix type liquid crystal display device displays an image in such a manner that an electric charge going in and out of each of several tens to millions of display pixels arranged in a matrix form is controlled by a switching element of each of the display pixels.
In the present specification, the display pixel indicates a device mainly constituted of a switching element, a pixel electrode connected to the switching element, a liquid crystal, and an opposite electrode disposed opposite to the pixel electrode through the liquid crystal. However, the display pixel in the case of a liquid crystal panel using IPS driving, the display pixel indicates a device mainly constituted of a switching element, a pixel electrode connected to the switching element, a liquid crystal, and a common electrode disposed on the same substrate.
In addition, the common potential in the present specification indicates the potential of the opposite electrode of the display pixel or the potential of the common electrode.
FIG. 2 is a schematic view showing a liquid crystal display device. FIG. 3A is a schematic structural view of an active matrix circuit in a liquid crystal panel 101 in FIG. 2.
In FIG. 2, the liquid crystal panel 101 includes a plurality of (N) scanning lines (corresponding to scanning lines A, B, C, D, . . . in FIG. 3A) extending in parallel to each other in the horizontal direction (lateral direction), a plurality of (M) signal lines (corresponding to signal lines (1), (2), (3), (4), . . . in FIG. 3A) extending in parallel to each other in the vertical direction (longitudinal direction) and crossing the scanning lines at right angles, M×N switching elements (TFTS etc.) respectively disposed in the vicinity of each of the crossing portions of the scanning lines and the signal lines, and a pixel electrode 111 connected to each of the switching elements.
In the liquid crystal panel 101, one end of the scanning line is connected to a gate electrode of each of the switching elements 110, and the other end is connected to a gate driver circuit 104 (scanning line driver circuit). On the other hand, one end of the signal line is connected to a source electrode of each of the switching elements 110 and the other end is connected to a source driver circuit 105 (signal line driver circuit).
FIG. 3B shows a display pattern (display pixels of four rows by six columns (A1 to D6)) as a part of a display region. FIG. 3B corresponds to the pixel electrodes 111 in FIG. 3A. That is, the display pixel A1 is mainly constituted of the switching element 110 disposed at the crossing point of the scanning line A and the signal line (1) in FIG. 3A, the pixel electrode 111 connected to the switching element, an opposite electrode provided opposite to the pixel electrode, and a liquid crystal existing between the pixel electrode and the opposite electrode.
For simplification, FIG. 3 shows only the scanning lines A to D, the signal lines (1) to (6), and the display pixels of four rows by six columns (A1 to D6) forming a part of the display region.
A typical example of display operation of the panel will be described in brief with reference to FIGS. 3A and 3B.
First, in accordance with a signal from a shift register circuit or the like (not shown) in the source driver circuit, only a part (pixel A1) of the lateral direction (horizontal direction) line of picture information (panel input image signal 203) is selectively sampled in the signal line (1), and its signal potential is applied to the entire of the signal line (1). Then a signal potential (turning on the TFT disposed in the vicinity of the crossing place) is applied only to the scanning line A. Only the switching element disposed in the vicinity of the place where the signal line (1) and the scanning line A cross with each other is turned on, so that the signal potential of the signal line (1) is applied to the pixel electrode. The liquid crystal is driven by the applied signal potential and the amount of transmitted light is controlled, so that a part (picture corresponding to A1) of the picture information is displayed on the display pixel A1.
Next, while the state in which the display pixel A1 displays is kept by an auxiliary capacitance or the like, at the next instant, only a part of the lateral direction (horizontal direction) line of the image signal is selectively sampled, and its signal potential is applied to the signal line (2) adjacent to the signal line (1). In this way, similarly to the display pixel A1, a part (picture corresponding to A2) of the picture information is displayed on the display pixel A2.
Such a display operation is sequentially carried out, so that a part (A1, A2, A3, A4, . . . ) of the picture information is sequentially displayed on the first pixel row (row A) in the lateral direction. During this, the scanning line A is applied with a signal which turns on the switching element disposed in the vicinity of each of the places where the scanning line crosses the signal lines.
Subsequently, when writing in all pixels of the first pixel row A in the lateral direction is ended, a signal potential (turning on a switching element disposed in the vicinity of a crossing place) is applied only to the scanning line B. Only a part (pixel B1) of the image signal is sampled in the signal line (1) and its signal potential is held. In the same way, only the pixel row (row B) corresponding to the second row in the lateral direction is sequentially written. Such a display operation is carried out by the number of pixel rows (N rows), so that one picture (frame) is displayed on the display region.
In addition, after one picture (frame) is displayed, in the liquid crystal display using TFTs or the like as switching elements, in order to prevent deterioration of the liquid crystal material, to eliminate display blur, and to keep display quality, signal potentials in which positive and negative polarities are inverted in one frame (one picture) are normally applied (alternating current driving) to the respective display pixels, while common potential is used as a reference.
These display operations are sequentially repeated and a plurality of pictures are obtained, so that images are displayed on the display region 106.
Next, the alternating current driving method briefly described in the above will be described in more detail. Incidentally, polarity patterns of display pixels (four rows by six columns) in conventional typical alternating current driving methods are shown in FIGS. 15A to 15B and FIG. 16A. The polarity patterns of FIGS. 15A and 15B and FIG. 16A correspond to the display pattern (display pixels of four rows by six columns (A1 to D6)) shown in FIG. 2B.
In the drawings (FIG. 1, FIGS. 15A and 15B, FIG. 16A, and FIG. 17A) showing polarity patterns in the present specification, the common potential is made a reference, and in the case where a signal potential applied to a display pixel is positive, “+” is shown, and in the case of negative, “−” is shown.
In addition, as a scanning system, there is interlaced scanning in which scanning lines of one picture (one frame) are divided into two (two fields) and scanning is carried out, and non-interlaced scanning in which scanning lines are sequentially scanned from the above on the picture. Here, examples using the non-interlaced scanning will be mainly described.
In FIG. 15A showing a conventional example, the polarities of image signals applied to all display pixels are inverted every frame, so that this example is called frame inversion driving.
As shown in FIG. 15A, the feature of the frame inversion driving is that signal potentials having the same polarity are applied to all display pixels in one arbitrary frame so that a polarity pattern (1) (positive) is displayed, while the polarity of the signal potentials applied to all the display pixels is inverted into negative so that a polarity pattern (2) (negative) is displayed in the next frame. That is, when attention is paid only to the polarity pattern, the frame inversion driving is a driving method in which two kinds of polarity patterns (polarity pattern (1) and polarity pattern (2)) are repeatedly displayed.
The problem of the conventional frame inversion driving is that a polarity inversion period is as long as one frame, and it becomes a frequency range (about 30 Hz) which can be recognized by a human eye, so that an observer can recognize, as flicker, a subtle difference between the display (1) at the time when the polarity of the image signal is positive and the display (2) at the time when the polarity of the image signal is negative. Especially in halftone display, remarkable flicker is observed.
Another conventional example shown in FIG. 16A is called source line inversion driving.
As shown in FIG. 16A, the feature of the source line inversion driving is that each of the display pixels is applied with a signal potential having a polarity opposite to a signal potential of its adjacent display pixel in the lateral direction (horizontal direction). In one arbitrary frame writing period, image signals having a signal potential of the same polarity (positive) with each other are applied to the display pixels (odd columns) expressed by A1, B1, C1, . . . , A3, B3, C3, . . . , A5, B5, C5, . . . . On the other hand, image signals having a signal potential of the same polarity (negative) with each other are applied to the display pixels (even columns) expressed by A2, B2, C2, . . . , A4, B4, C4, . . . , A6, B6, C6, . . . . In this way, a polarity pattern (1) is displayed. Then, in a next frame writing period, an image signal having the polarity opposite to the polarity pattern (1) displayed in the proximate frame writing period is applied to each of the display pixels so that a polarity pattern (2) is displayed.
That is, as shown in FIG. 16A, similarly to the conventional frame inversion driving, the conventional source line inversion driving is also a driving method in which two kinds of polarity patterns (polarity pattern (1) and polarity pattern (2)) are repeatedly displayed.
FIG. 18 shows an example of a timing chart of a panel input signal when the conventional source line inversion driving is used and a white picture is displayed on the display region of a liquid crystal panel which is normally black. The signal corresponds to the display pattern (display pixels of four rows by six columns (A1 to D6)) shown in FIG. 2B and FIG. 16A.
Another conventional example shown in FIG. 15B is called gate line inversion driving.
As shown in FIG. 15B, the feature of the gate line inversion driving is that each of the display pixels is applied with an image signal having a polarity opposite to its adjacent display pixel in the longitudinal (vertical) direction. In this method, the polarity of the signal potential of the image signal is inverted from positive to negative or from negative to positive every horizontal scanning period.
That is, similarly to the conventional driving method, this is a driving method in which two kinds of polarity patterns (polarity pattern (1) and polarity pattern (2)) are repeatedly displayed.
By this source line inversion driving and gate line inversion driving, flicker which is a problem in the frame inversion driving is reduced. However, the problem of the source line inversion driving and the gate line inversion driving is that since a stripe called disclination is produced between adjacent display pixels applied with opposite polarities, so that the brightness of the entire display picture is lowered.
In the present specification, the disclination means poor display (light loss in the case of normally white display, light leak in the case of normally black display) due to disturbance of an oriented state of liquid crystal caused by the potential difference which is produced between the display pixel applied with the image signal having the positive polarity and the display pixel applied with the image signal having the negative polarity.
The potential difference between the adjacent display pixels is produced from electric flux lines shown in FIGS. 14(1) and 14(2). FIG. 14(1) is a top view showing the state of electric flux lines produced between two pixel electrodes (1) and (2) when an effective electric field (positive or negative) is applied to the pixel electrodes (1) and (2) in the direction vertical to the paper surface. FIG. 14(2) is a sectional view. However, for convenience, FIG. 14(1) shows only the electric flux lines produced in the lateral direction between the pixel electrodes (1) and (2), and FIG. 14(2) shows the state of the electric flux lines immediately before liquid crystal molecules oriented in the vertical direction respond to the application of the electric field.
FIG. 16B shows a disclination pattern corresponding to FIG. 16A. In FIG. 16B, the disclination is formed at a fixed position, and although the polarities of the signal potentials applied to the display pixels are different, the disclination pattern (1) is substantially the same as the disclination pattern (2).
In addition, although not shown, as another alternating current driving method, there is proposed an alternating current driving method (dot inversion driving) in which the polarity of an image signal is inverted for every writing of all adjacent display pixel and the inverted signal is applied to the display pixel. In the dot inversion driving, the polarities of adjacent pixels are different from each other, so that the influence of a potential difference produced between the adjacent display pixels is great, and the disclination greatly influences the display.
As described above, in the conventional alternating current driving methods (source line inversion driving and gate line inversion driving), like the example shown in FIGS. 16A and 16B, the polarity pattern (1) and the polarity pattern (2) are repeatedly displayed, and the disclination is continuously formed at the fixed position between adjacent display pixels having different polarities, so that the brightness of the picture is lowered. In addition, the same can be said of another alternating current driving method (dot inversion driving).
In another alternating current driving method (frame inversion driving), although the disclination is not produced, flicker is produced.
The number of display pixels of a display has been increasing year after year, and a driving frequency becomes very high for a panel with a large number of display pixels. For example, it is said that the NTSC standard requires about 400 thousand display pixels, and the HDTV standard requires about 2 million display pixels. Thus, the maximum frequency of an input image signal is about 6 MHz in the NTSC standard, and about 20 MHz to 30 MHz in the HDTV standard. In order to accurately display this image signal, a clock signal is required to have a frequency (for example, about 50 MHz to 60 MHz) several times that of this image signal. In future, it is expected that display of high fineness and high picture quality is increasingly required, and an image signal with a very fast dot clock is to be treated.
Hitherto, it has been difficult to accurately make alternating current of an image signal and a clock signal having such a high frequency band range and to drive a liquid crystal panel. This is because a liquid crystal material used in a conventional LCD has a slow speed (several tens ms to hundreds ms) of response from application of a potential, and even if a driver circuit is constituted of TFTs which use, for example, amorphous silicon or polycrystalline silicon and can operate in a high frequency band region, the liquid crystal material can not respond to the high speed operation, which is a problem.